Abstract
Contactless manipulation of matter is essential for studying physical phenomena. Acoustic manipulation of liquid samples using ultrasonic phased arrays provides a novel and attractive solution for mid-air manipulation, such as levitation, transportation, coalescence, mixing, separation, evaporation, and extraction, with a simple and single sequence. Despite the importance of gravity in droplet dynamics, its effect on a levitated droplet with an ultrasonic phased array remains unclear. To disseminate acoustic manipulation, better understanding of the fundamental physics of a droplet manipulated by ultrasonic phased arrays is required. Here, we show contactless levitation, transportation, and coalescence of multiple droplets under both ground and reduced gravity. Under ground gravity, the possible levitation size of the sample is limited to below the half wavelength of sound. Under reduced gravity, however, droplets that are larger than the limit can be successfully levitated, transported, and coalesced. Furthermore, the threshold of sound pressure for droplet levitation and manipulation could be minimised with the suppression of nonlinear acoustic phenomena under reduced gravity. These insights promote the development of contactless manipulation techniques of droplets for future space experiment and inhabitancy.
Highlights
Contactless manipulation of matter is essential for studying physical phenomena
The theoretical and experimental lower limits of stable levitation under reduced gravity (pmin(μG) in Fig. 3c) can be expanded, the allowable diameter of the sample has been considered to be limited to the half wavelength or less (Fig. 4a)
The experimental results show that the droplets that are larger than the half wavelength can be levitated, transported, and coalesced under the reduced gravity environment
Summary
Contactless manipulation of matter is essential for studying physical phenomena. Acoustic manipulation of liquid samples using ultrasonic phased arrays provides a novel and attractive solution for mid-air manipulation, such as levitation, transportation, coalescence, mixing, separation, evaporation, and extraction, with a simple and single sequence. Better understanding of the fundamental physics of a droplet manipulated by ultrasonic phased arrays is required. The threshold of sound pressure for droplet levitation and manipulation could be minimised with the suppression of nonlinear acoustic phenomena under reduced gravity. These insights promote the development of contactless manipulation techniques of droplets for future space experiment and inhabitancy. It is of necessity and importance to demonstrate the feasibility study of droplet manipulation under lower gravity to obtain physical insights for the further development of ALM. We demonstrate a feasibility study and fundamental physics for the levitation, transportation, and coalescence of droplets using an ultrasonic phased array under varying gravity. Our insights could pave the way towards droplet manipulation for lab-in-a-drop applications both on Earth and in space
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